1. Field of the Invention
The present invention relates to a development device employed in an image forming apparatus using an electrophotography such as an office machine, for example, a laser beam printer, a facsimile machine, a digital copier, etc., to develop an electrostatic latent image formed on an organic photoconductive drum into a toner image. More particularly, the present invention relates to a development device to detect a developing gap between a developer conveying body such as a developing roller and a photoconductive drum accurately and economically.
2. Description of the Related Art
FIG. 1 is a schematic view showing a general development device.
The development device 1 as shown in FIG. 1 includes an organic photoconductive drum 4 on which an electrostatic latent image is formed by a LSU (Laser Scan Unit) (not shown) using an electric potential of a surface thereof, a charging roller 2 to rotate in contact with the organic photoconductive drum 4 to electrically charge a surface of the organic photoconductive drum 4, and a developer conveying body 5 (hereinafter referred to as “developing roller”) to rotate opposite to the organic photoconductive drum 4, and to add a developer made of a predetermined color toner to the electrostatic latent image formed on the organic photoconductive drum 4 to form a visible image.
The developing device also includes a developer supplying roller 6 to supply the developer to the developing roller 5, a developer amount regulating member 7 to regulate a developer layer formed on the developing roller 5, a cleaning blade 10 to remove a remainder developer that remains on the surface of the organic photoconductive drum 4 after the organic photoconductive drum 4 is rotated in one cycle, and a power supply unit 20 to supply a power to the organic photoconductive drum 4, the developing roller 5, and the developer amount regulating member 7.
Hereinbelow, an operation of an image forming apparatus having the development device 1 as constructed above will be described.
The charging roller 2 electrically and uniformly charges the surface of the organic photoconductive drum 4 to a predetermined voltage. Afterwards, the LSU converts a digital signal input from a computer or a scanner to an optical signal in a form of a laser beam through a laser diode. The LSU then emits the optical signal onto the organic photoconductive drum 4, thereby forming the electrostatic latent image on the organic photoconductive drum 4.
The developer supplied to the developing roller 5 is conveyed to a developing gap G between the organic photoconductive drum 4 and the developing roller 5 while the developing roller 5 is rotated. At this point, the developer amount regulating member 7 disposed above the developing roller 5 maintains a toner layer of the developer on a surface of the developing roller 5 at a predetermined thickness.
As the organic photoconductive drum 4 is rotated, the developer 8 jumps onto the electrostatic latent image formed on the organic photoconductive drum 4 due to an electrical potential difference that occurs between the electrostatic latent image and the surface of the developing roller 5, thereby developing the electrostatic latent image formed on the surface of the organic photoconductive drum 4 into a visible toner image.
Meanwhile, when incoming paper enters between the organic photoconductive drum 4 on which the toner image is formed and a transfer roller (not shown) disposed at a lower portion of the organic photoconductive drum 4, the transfer roller transfers the toner image from the organic photoconductive drum 4 to the paper.
While the organic photoconductive drum 4 is continuously rotated, the cleaning blade 10 removes the remainder developer from the surface of the organic photoconductive drum 4, enabling the next electrostatic latent image to be formed on the organic photoconductive drum 4. The toner image is settled down on the paper by heat and pressure and is then discharged out of the development device 1 so that a series of image forming processes are completed.
For the image forming apparatus as described above, it is important to maintain a constant developing gap G between the organic photoconductive drum 4 and the developing roller 5 because the constant developing gap G guarantees uniform and stable developing quality during the process of developing the electrostatic latent image on the organic photoconductive drum 4 into the toner image using the developer 8.
FIG. 2 is a front view showing a development device 1 of a conventional image forming apparatus. As shown in FIG. 2, the development device has a spacer 5a including two spacer rolls that are disposed at both ends of a shaft 5b of the developing roller 5, which are rotated in contact with the surface of the organic photoconductive drum 4. The spacer 5a is provided to maintain a predetermined developing gap G between the organic photoconductive drum 4 and the developing roller 5.
Since the spacer 5a has an outer diameter that is larger than an outer diameter of the developing roller 5, the organic photoconductive drum 4 and the developing roller 5 opposing each other that are respectively rotated by an organic photoconductive drum gear 4a and a developing roller gear 5c at a predetermined linear velocity rate, maintain the predetermined developing gap G therebetween. When the developing gap G is expressed by an outer diameter D1 of the developing roller 5, an outer diameter D2 of the spacer 5a, an outer diameter D3 of the shaft 5b, and an inner diameter D4 of the spacer 5a, the developing gap G is defined by (D2-D1)/2-(D4-D3)/2.
However, there still occurs a variation in the developing gap G due to a precision in the sizes of the above-related components. A variation of the developing gap G results in a deteriorated image quality. That is, as the developing gap G becomes larger, a developing electric field becomes weaker and thus, an image density becomes lowered. As the developing gap G becomes narrower, the developing electric filed becomes stronger, subsequently increasing the image density, and even worse, causing electric discharge and subsequent image noise. Therefore, in order to solve these problems, the development device has to detect a predetermined developing gap and properly vary the developing electric field according to the detected developing gap.
Accordingly, a technology has been suggested to detect the image density using an optical sensor based on a reference image previously formed on the organic photoconductive drum or transfer belt. However, fabrication costs are high due to a cost of the optical sensor.
U.S. Pat. No. 5,521,683 discloses a technology that detects a developing gap by supplying a constant voltage and a constant current, which has a lowered accuracy since voltage change and current change are frequently made due to the variation of the developing gap.